1. Field of the Invention
The present invention relates to a piezoelectric bending transducer and, more particularly, to a flat piezoelectric bending transducer adapted for incorporation into a module comprising a plurality of such transducers.
2. Description of Related Art
Piezoelectric bending transducers are used in applications that can exploit the indirect or reciprocal piezoelectric effect, that is, the transformation of electrical energy into mechanical energy. A piezoelectric element of this type is equally suited, however, for transforming mechanical energy into electrical energy by exploiting the direct piezoelectric effect.
There are many technical applications for piezoelectric bending transducers. For example, they can be used in a piezoelectric print head for an ink-jet printer, or such a transducer can be used as a sensor for the measurement of acceleration or pressure, as an actuating element for transmitting pattern information in a weaving or knitting machine, and as an actuating element in a line of Braille in a reader for the blind, in a pneumatic valve, in a recording measuring instrument or in a contactless surface-measuring instrument, just to name a few uses.
Flat piezoelectric bending transducers are shown in DE 195 20 796 A1, DE 40 25 436 A1 (U.S. Pat. No. 5,404,067) and DE 196 20 826 B1. In such transducers, the application of a voltage between an outer contact area and an inner contact area causes contraction or expansion of a piezoelectric coating, depending on its polarity, so that the entire transducer bends or so that, if one end of the transducer is held fixed, the other end is deflected in a defined way. Conversely, a voltage between the inner contact area and the outer contact area is produced if the bending transducer is mechanically deformed from a neutral rest position.
Control modules including plural piezoelectric transducers are shown in U.S. Pat. No. 3,961,501 and EP 210 790 A2. Such a module, having a plurality of piezoelectric bending transducers, can be used for converting an electronic information signal into a mechanical deformation that can be sensed. For example, a module with a multiplicity of electrical bending transducers arranged in parallel can be used for driving needles or loop-forming elements of a weaving or knitting machine. For pattern forming in the goods produced by a textile machine, the information as to whether or not a loop is to be formed is obtained by mechanically sensing the deflection of a corresponding piezoelectric bending transducer.
In the case of both a piezoelectric bending transducer and a module with a plurality of such transducers, an electrical connection must be provided to contact areas on the piezoelectric coating. DE 40 25 436 A1 forms an outer contact area of the piezoelectric coating as a thin metal layer, and provides an electrical connection to the coating by soldering a corresponding connecting wire onto the metal layer. For electrically connecting the inner contact area, DE 40 25 436 A1 discloses placing a metal platelet between the piezoelectric coating and a supporting body for the coating, and then soldering a connecting wire to the platelet. Thus, electrical connections with the contact areas take place via a soldered contact and a corresponding connecting wire.
However, that is not an optimum arrangement because the operation of soldering a connecting wire onto a relatively small component such as a piezoelectric bending transducer is difficult to automate, and that therefore incurs disadvantageously high production costs. In addition, a high level of reliability is required for most applications of piezoelectric bending transducers, and in particular for modules made up of a plurality of such transducers. Both the soldered contact and the connecting wire represent points of potential failure, thus reducing the reliability of the transducer and therefore of the module. Moreover, replacing a built-in bending transducer or a built-in module with conventional soldered contacts is relatively complicated.
DE 36 34 484 A1 proposes that the electrical connections to the transducer be via conductive rubber. For this purpose, the conductive rubber is permanently clamped between a housing and the transducer. However, that is still not an optimum arrangement for some applications.